My friend RON, the Research Octane Number

[BruceT]

A few people have asked about whether to use standard ULP (91 RON), premium (PULP, 95 RON), or 98 RON. I'll give my view based on my studies in chemical engineering and some reading.

What RON is not:
RON is not a measure of the energy content of a fuel. So there is nothing intrinsically more efficient or powerful about using 98 RON vs 91 RON. In most Japanese and Aussie cars you will notice no difference.

What it is:
RON is the proportion of iso-octane (0-100%) which, when mixed with n-pentane (100-0%) will give the same knocking characteristics as the fuel being tested. Eg. a 91 RON fuel will have the same knocking characteristics as 91% iso-octane mixed with 9% n-pentane.

What is knocking:
A combustible mix of fuel and air can burn 2 ways:
1) smoothly via a flame front - ideal, gives smooth power delivery.
2) explosively like a detonation - which gives a characteristic "pinging" sound called "knocking", damages the engine over time, and gives poor performance. (By the way, diesel fuel burns this way all the time, that's why diesels sound awful.)

Until recently engines were designed for a minimum RON fuel and that was that. As 95 RON is the widespread minimum in Europe, most European cars are built for 95 RON. The WP, WQ Fiesta require 95 RON. There is NO advantage to using 98 RON in such cars, except under heavy load, low temperatures (startup) and a few other conditions.

Recently, engines have been given knock sensors designed to detect the onset of knocking and adjust timing (& perhaps fuel mix & other things) to compensate. See:
http://wiki.answers.com/Q/What_does_a_knock_sensor_do

So what's up with the Aussie WS Fiesta?
I've been running my WS Zetec on 91 RON with no sign of knocking and smooth power delivery. Ford recommends any fuel from 91-98 RON:
http://www.ford.com.au/servlet/Conte...=FOA&c=DFYPage
But the run the mouse over footnote 10 on the same site and it says:
"For optimum performance use fuel of 95 RON or higher"

On this site when they compare the Fiesta to other cars:
http://www.ford.com.au/servlet/Conte...88217&site=FOA
They have a footnote that states (press MORE), "All performance data measured using 95 RON fuel."

So my conclusion is: If you want your full 88kW and best fuel economy then use 95 RON fuel. There is no extra benefit from 98 RON fuel. 91 RON fuel will do the car NO harm whatsoever as the engine will adjust to the lower fuel RON, but in doing this some performance (how much?) will be lost. I've no idea whether the loss of efficiency with 91 RON makes 95 RON more cost effective. I guess you can only experiment - I will.

It would be great if Ford had a little more technical information out there!
:

[dannyhilton]

My WP is gutless, like really gutless with a captial G, when running 91RON. Put 98RON in it and it revvs better, pickup is massivly improved, consumption drops by about half a litre, it does the world of good. I have not put 91RON in my WS, but I am planning a big trip so I'll see about 91RON. Magazines have done tests, and based in my experience 98RON is superior. I find PB Ultimate the best. Meh, work pays for my fuel and my BP fuel card makes me biased...

[BruceT]

I'm pretty sure the WP Fiesta has a 95RON engine:
http://www.drive.com.au/Editorial/Ar...leID=8105&vf=1
http://www.drive.com.au/Editorial/Ar...leID=8063&vf=1
Although reviews differ:
http://www.car4me.com.au/auto-review...Q~OpenDocument

It may be that the engine is less successful atr adapting to lower RON fuel.

My WS is zippy on regular 91 RON ULP provided revs are kept above 3000. One -just once - I went from a standing start, changed up to 2nd at 4000 RPM and floored it to the red line in 2nd to get away from some twirp. I was surprised how quickly 100 km/h came up - along with 6500 rpm (I was surprised). Slightly slower than our V6 Magna that does it in about 9.5 secs.

I have no doubt your car runs better on 98 RON vs 91. I question whether you would lose anything with 95 RON though.

[dannyhilton]

When fuel prices were high last year we were told we could not use 98RON due to the added expence. I used 95RON and even then, I still feel 98RON is superior. And not just because of all the fancy branding, it just gives the Fez a more enthuastic engine from both 91/95RON. I pressed the old WP very hard, so I was at the extreme of the scale in my findings.

[LethalLeigh]

With the WP I owned, it ran like a pig on 91RON, very twitchy and unpredictable on takeoff and not very smooth, especially noticeable with the A/C on. These days I run both of my cars on 98RON for piece of mind.

[dannyhilton]

Good point Leigh. With the aircon on using 98RON my WP still went well, but on 91RON is was gutless below four grand and only had decent power till five grand.

BP have done various tests, as have indipendent magazines, and they all agree 98 is a better alternative. Especially in an already underpowered car, but the benifits also translate into large cars.